1. Field of the Invention
The present invention relates to a solid-state image pickup device, and in particular, to a solid-state color image pickup device having a photosensitive semiconductor layer sensitive to a light of a specific wavelength range, thereby producing video signals of separated colors.
2. Description of the Prior Art
A solid-state color image pickup device of this type is described in Japanese Patent Laid-Open Publication Nos. 103167/1983, 103165/1983 and 103166/1983.
The Japanese Patent Laid-Open Publication No. 103167/1983 discloses a solid-state color image pickup device comprising a photosensitive semiconductor layer disposed on a primary surface of a semiconductor substrate and being sensitive only to a light having a wavelength not exceeding the wavelengths of blue (B) and a magenta (M) filter configured in a shape of stripes on the photosensitive semiconductor layer so as to form MOS switching gates and photodiodes in the principal surface of the substrate. A photosensitive cell of a pixel is constructed with three MOS switching gates and two photodiodes.
Transparent planar electrodes fixed on the principal surface of the substrate are subjected to a bias voltage to collect photocarrier excited in the layer with an incident light. MOS switching gates are connected to one of the transparent electrode to deliver a blue component of a video signal. A photodiode below the stripe filter supplies a red (R) component of a video signal. This is because the light incident to the photodiode is a red light which has passed through the magenta filter and the photosensitive semiconductor layer. The other photodiode delivers a yellow (Y) component of a video signal. This is because the blue light component of the light incident to this photodiode has been removed by the photosensitive semiconductor layer.
The Japanese Patent Laid-Open Publication No. 103165/1983 describes a solid-state color image pickup device comprising three layers of photosensitive semiconductor in a structure similar to that of the Japanese Patent Laid-Open Publication No. 103167/1983. Among the three semiconductor layers, the layer next to the substrate is sensitive to lights of any wavelengths, the intermediate layer is responsive to a light with a wavelength not exceeding the wavelengths of green, and the further-most layer is sensitive to a light whose wavelength is in the wavelength range of blue or therebelow. The device is not provided with the stripe filter; moreover, the photodiodes are not formed on the primary surface of the substrate. Three MOS switching gates are connected to the associated transparent photosensitive semiconductor electrode layers. This provision enables the R, G and B color video signals to be fed from the respective switching gates.
In the Japanese Patent Laid-Open Publication No. 103166/1983, there is disclosed a solid-state color image pickup device having two photosensitive semiconductor layers. This structure is considered intermediate between the structures of image pickup devices disclosed by the Japanese Patent Laid-Open Publication Nos. 103167/1983 and 103165/1983. In the structure, the semiconductor layer next to the substrate is sensitive to a light with a wavelength not exceeding the green light wavelengths, whereas the other layer is responsive to a light having a wavelength in the blue light wavelength range or therebelow. This device is not provided with the stripe filter but a photodiode is formed for each pixel on the principal surface of the substrate, and two MOS switching gates are connected to the associated transparent photosensitive semiconductor electrode layers. With this configuration, the two switching gates respectively supply the G and B color video signals, whereas the photodiode delivers the R color video signal.
In any devices described above, the bias voltage applied to the two planar transparent electrode layers causes an electric field to be established in the photosensitive semiconductor layer in a direction perpendicular thereto, which leads to the following difficulties in the image pickup device.
For example, if the photosensitive semiconductor contains a pinhole which hinders generation of an electric field, a video signal delivered from the photosensitive cell causes the associated pixel to be displayed as a blank or white dot in a reproduced picture. In addition, a semiconductor material having a resistivity less than about 10.sup.10 .OMEGA.cm cannot be utilized for the photosensitive semiconductor layer because of its large dark current, which prevents a desirable low noise level from being developed. Consequently, the freedom of selection of semiconductor materials is limited. Furthermore, the electrode layer below the photosensitive semiconductor layer has also to be transparent, that is, a transparent electrode material such as ITO is necessarily be used, which limits the freedom of selection of electrode materials. Moreover, the multilayer structure of these devices necessitates a great number of processes and complicated processing in manufacturing.